Conventionally, in the case of a mammographic apparatus of the X-ray radiographic type for a breast cancer checkup used in a medical institution such as a hospital, whether there is breast cancer is checked by acquiring X-ray photo images, such as transmission X-ray photos or X-ray transmission images, which provide anatomical information on patients' breast areas. Only by checking X-ray photo images, a sufficient breast cancer checkup cannot necessarily be carried out (see Nonpatent Document 1).
Then, it has been proposed to perform a breast cancer checkup by using the PET (positron emission tomography) technique which can provide RI distribution images reflecting a body distribution of a radioisotope introduced into a patient, and photographing RI distribution images of the patient's breast area (see Nonpatent Document 2). Unlike X-ray transmission images which provide anatomical information, RI distribution images provide biofunctional information, and may therefore enable detection of breast cancer overlooked by means of X-ray transmission images.
An apparatus of the PET type, as shown in FIG. 13, includes a top board 91 for supporting a patient M lying on her back or on her side, and a gantry 92 having an opening (tunnel) 92A formed centrally thereof for allowing the top board 91 with the patient M placed thereon to move in and out. A large-sized gantry 92, as shown in FIG. 14, has a ring type gamma-ray detector 93 arranged therein. When RI distribution images are acquired with the conventional apparatus, the gamma-ray detector 93 detects gamma rays (annihilation gamma-rays) of 511 keV energy generated by an RI having reached the breast area Ma after being introduced into the patient M placed on the top board 91 and having entered the opening 92A of gantry 92. On the other hand, at stages downstream of the gamma-ray detector 93, emission data for RI distribution image acquisition are collected according to gamma ray detection signals outputted from the gamma-ray detector 93, and reconstruction processing is carried out based on the emission data collected, to acquire RI distribution images of the breast area Ma of the patient M.
[Nonpatent Document 1]    Radiology 2005; 234: 527-534.
[Nonpatent Document 2]    Nuclear Instrument and Methods in Physics Research A 527 (2004) 82-91